FIG. 14 shows an optical communications network 1100 in which 10/100 BASE-TX signals having 32 channels are multiplexed by code division multiplexing, or CDM.
The optical communications network 1100 is a WDM-CDM-PON system, that is, a passive optical network (PON) using a combination of wavelength division multiplexing (WDM) technology and code division multiplexing (CDM) technology. An optical line terminal (OLT) 1200 is placed in a central office 1110, which is close to a key network, and an optical network unit (ONU) 1300 is placed on the subscriber side. A wavelength filter 1120 is provided on the ONU 1300 side, and the central office 1110 and the wavelength filter 1120 are connected by an optical fiber 1130. WDM-CDM signals sent from the central office 1110 to the ONU 1300 are branched at the wavelength filter 1120 for each wavelength, which is then sent to an optical coupler 1400.
Multiple ONUs, represented by the ONU 1300, are connected to the optical coupler 1400. For purposes of discussion, the 32 ONUs connected to the optical coupler 1400 and a single OLT 1200 constitutes one group.
The same wavelength is used for upstream communication within a group from ONU 1300 to OLT 1200 and downstream communication from the OLT 1200 to the ONU 1300 using CDM technology. A different wavelength is allotted to each group, and wavelengths are separated or multiplexed at the wavelength filter 1120 and an intra-office filter 1122 provided inside the central office 1110 in each group.
The OLT 1200 includes an interface 1210. Between the interface 1210 and the key network, packets are transmitted and received, frames generated and packets extracted. There are 32 types of codes that are allotted to encoders 1232-1˜32. The encoders 1232-1˜32 convert downstream frames into code spread signals. The code spread signals are added by a multiplexed adder 1240 to generate code division multiplexing (CDM) signals. The CDM signals are converted into CDM optical signals by an optical module 1250 and then transmitted to the ONU 1300. The CDM optical signals generated at each OLT 1200 are wavelength-multiplexed by the intra-office filter 1122 and then sent to the ONU 1300 as WDM-CDM signals.
The wavelength filter 1120 separates WDM-CDM signals, and each of the separated CDM optical signals is sent to the optical coupler 1400. The CDM optical signals are branched into 32 optical signals by the optical coupler 1400 and sent to each ONU 1300.
At each ONU 1300, the optical module 1350 converts CDM optical signals into CDM electric signals, which are then sent to a decoder 1382, composed of a CCD matched filter 1384 and a comparator 1386.
The CCD matched filter 1384 convolutes codes allotted to the decoder 1382 for CDM electric signals. The comparator 1386 reproduces a downstream frame based on the convoluted result at the CCD matched filter 1384. Afterwards, a packet extracted from the frame is sent to a user terminal via an interface 1310.
On the other hand, signals from a user terminal are received by the interface 1310 of the ONU 1300, encoded by the encoder 1332 and then converted to optical signals at the optical module 1350. These optical signals are multiplexed at the optical coupler 1400 to become CDM optical signals, wavelength-multiplexed at the wavelength filter 1120 and then sent to the central office 1110 as WDM-CDM signals. These WDM-CDM signals are wavelength-separated to CDM optical signals by the central office filter 1122 and then sent to the OLT 1200.
The OLT 1200 converts the CDM optical signals to CDM electrical signals at the optical module 1250, splits the electric signals at a splitter 1270 and then sends them to decoders 1282-1˜32. The decoders 1282-1˜32, which are each composed of a CCD matched filter and a comparator like the decoder 1382 in the ONU 1300, reproduce an upstream frame from an electric signal. The upstream packet is transmitted to the key network via the interface 1210.
In theory, components (cross correlation components) from another channel become zero at the time of decoding on the receiving side in the CDM communication using an orthogonal code set. In practice, however, they do not become completely zero due to the imperfection of a device or the like. Accordingly, if the light intensity of another channel is large relative to that of the object channel to be decoded, the cross correlation components may cause a large amount of noise, which prevents the extraction of autocorrelation components.
The attenuation of an optical fiber is about 0.5 dB/km. If a distance from an optical demultiplexer is different by 10 km between ONU-1 and ONU-2, for example, light intensity differs by about 5 dB. In the case that a coverage (dynamic range or reception range) is about 5 dB on the receiving side, a downstream optical signal received by ONU and an upstream optical signal received by OLT depart from the dynamic range if a distance from the optical demultiplexer is 10 km or more. As a result, an optical signal cannot be received normally.
In view of the above, there is a need for making the optical intensity of an upstream optical signal transmitted from each ONU constant at the time of multiplexing at an optical directional coupler by providing a variable optical attenuator that is common for an upstream optical signal and a downstream optical signal, giving the same attenuation to the upstream optical signal and the downstream optical signal and controlling the attenuation in the variable optical attenuator by an OLT. Moreover, there is a need to contain optical intensity of a downstream optical signal received by an ONU and an upstream optical signal received by an OLT within a dynamic range.